An important step in investment-type metal casting operations, where parts are formed directly by casting molten metal into appropriate molds, is the filtering of inclusions such as refractory particles, dross and the like from the molten metal.
U.S. Pat. No. 3,893,917 to Pryor et al. describes molten metal filters of a foamed ceramic structure. The filters are fabricated by providing an open cell, hydrophilic flexible organic foam material having a plurality of interconnected voids and impregnating material with an aqueous ceramic slurry so that the web is coated. After excess slurry is removed from the foam, it is dried and heated to burn out the foam leaving a ceramic foam shell. Molten metal filters may be made in this way from a wide variety of ceramic materials including alumina, chromia, zirconia, magnesia, silica and titanium dioxide. One major problem associated with such filters is their fragility. Typically this limits the maximum size in which they can be prepared and used. Moreover, because such filters rely on internal interconnected open porosity of irregular size and distribution for filtering molten metal, flow rates through such filters are relatively low. In order to cast a sufficient amount of molten metal before the metal has had an opportunity to cool and solidify, it is often necessary to employ a plurality of foam filters each positioned between an individual mold and a common casting ladle serving many molds. This greatly complicates mold construction procedure.
The use of molten metal filters of honeycomb structure is also well known. Because of their relatively massive size, they have been formed in the past from extremely low expansion material such as cordierite and zirconia spinel to provide adequate protection from damage due to thermal shock occurring in metal filtering operations.